Power consumption is always of concern in aircraft. Power that is consumed in generating hydraulic fluid under pressure, electrical energy, etc. reduces the power available for propulsion and/or limits aircraft endurance as a result of the consumption of fuel in the operation of various hydraulic systems that otherwise might be expended in propelling the aircraft. Not untypically, in aircraft of more than basic simplicity, hydraulic systems are used for control purposes to change the aerodynamic configuration of the aircraft. In the usual case, hydraulic motors are mechanically connected to a control surface for the aircraft and through appropriate servo mechanisms, caused to operate to vary the aerodynamic configuration to some commanded configuration.
While such systems work well, when being operated under less than peak load, the volumetric flow demand of the hydraulic system may be considerably in excess of that required to meet the aerodynamic load. Consequently, power is consumed in unnecessarily pressurizing an unnecessarily large quantity of hydraulic fluid.
In an effort to avoid this and associated problems, it has been proposed to use variable displacement hydraulic motors in such aircraft systems. See, for example, U.S. Pat. No. 4,907,408 issued Mar. 13, 1990 to Barker.
When variable displacement hydraulic motors are employed, their displacement may be reduced for relatively light loads, thus requiring a lesser volumetric flow rate of hydraulic fluid in order to make the required system adjustment. However, in many hydraulic systems intended for aircraft, there is the possibility of experiencing a so-called aiding load. In such a case, aerodynamic forces acting on the control surface to be adjusted are aiding the system as it moves the surface. This, in turn, requires that the hydraulic motor act as a pump in order to maintain control of the position and the speed of the aircraft control surface. With prior art control schemes, in which the system modulates towards a higher displacement of the hydraulic motor as higher aiding loads occur, system instability may result and difficulty in properly moving the aerodynamic control surface to the desired position can occur.
Another difficulty occurs where the hydraulic system utilizes a variable pressure source. In such a case, the prior art has typically optimized the system to one particular pressure, namely, the pressure at which the system would most likely be operated. As a consequence of this optimization, operation at other pressures then becomes inefficient and undesirably increases power consumption.
The present invention is directed to overcoming the above problems.